function y= rootlocus_AB(x,p)
    rho=(x-1.0).*(x.^(p-1));
    switch p
    case 1
        sigma=1;
    case 2
        sigma=1.5.*x-0.5;
    case 3
        sigma=23/12*x.^2-16/12*x+5/12;
    case 4
       sigma=55/24.*x.^3-59/24.*x.^2+37/24.*x-9/24;
    end
    y=1.0*rho./sigma;
end

